An electrical connection box to be mounted on an automobile is configured to include an electronic component block and a casing for accommodating the electronic component block (for example, see Patent Document 1). The casing is configured to include a frame having a frame peripheral wall, and a cover. The cover is formed as a member that covers an opening portion surrounded by the edge of the frame peripheral wall. The casing is configured such that the opening portion is covered by rotational movement of the cover and the locking portions of the frame and the cover are fitted to each other to complete the assembly thereof protrusions are formed on the cover of the casing for rotational movement of the cover. On the frame peripheral wall, protrusion engaging portions are formed so as to be engaged when the protrusions of the cover are inserted.